Sandia LabNews

Synthetic sludges should help deal with deadly storage tanks

Sandia research group synthesizes nonradioactive substitute to aid nuclear waste clean-up

Synthetic goods are generally modeled on scarce but desirable materials — diamonds, fine wools, even fruit juices.

Jim Krumhansl’s offering to the world is a bit different. Jim (6118) has created synthetic sludge. Unappetizing, perhaps? You thought there was enough of the real thing? But the unusual product, which harmlessly mimics the deadly sludge found in nuclear waste storage tanks, could save taxpayers tens of millions of dollars — and maybe more — in cleanup costs around the United States. It will allow researchers to safely and cheaply determine which radioactive wastes will remain embedded in, and which will migrate from, the sludge found in waste storage tanks.

This should permit easy and relatively cheap decommissioning of some tanks, rather than "worst case" (and therefore expensive) disposal, with maximum safeguards for every tank.

The work, to be reported Aug. 23 at the American Chemical Society meeting in New Orleans, is an outgrowth of the February 1995 Galvin Report’s suggestion that national laboratories find new science to cut down on the very large costs of projected environmental cleanup bills.

Joint project

The sludges created by the research group led by Jim are part of a joint project among Sandia, Pacific Northwest National Laboratory (PNNL), and the University of Colorado. The sludges consist of "nonradioactive representations of extremely radioactive materials" that precipitate as a matter of course during storage in giant underground tanks that may contain up to a million gallons of nuclear waste, says Jim.

Naturally occurring sludge sticks to the walls of tanks and could serve as a long-term source of radioactive contamination in the environment.

"When the tanks are emptied, some of the sludge won’t budge," says Jim. "The tanks will be sluiced, sloshed, and squirted, but people won’t be sent inside to clean them up."

While purists might prefer that scientists experiment on real sludge, workers feel differently. Many of these materials are so radioactive that working with them is both costly and requires taking extreme measures to protect the health of researchers performing the work.

Synthetic sludges, while chemically similar to radioactive ones, are not radioactive and can be handled without danger by lab workers attempting to quantify how much radioactive material that the sludges will store or release, and for how long.

Costs of decommissioning the tanks, treating every material as a worst case in its potential to escape into the environment, could be hundreds of millions of dollars. "The decision how to treat these tanks ultimately depends on how much hazard there is from their residual radioactivity being able to move about. If virtually none of it goes any place, then you’re a lot freer to do simple decommissioning techniques," Jim says. "The question is what fraction of radionuclides in the sludge will stay there indefinitely and what fraction could become mobile and enter the groundwater."

Some will be tied up in solid materials that constitute the sludge, says Jim. Some should be stable indefinitely in an underground environment, even if left near the surface.

To judge whether the synthetic sludges are realistic representations of their more lethal counterparts, group member Jun Liu of PNNL has done transmission electron microscope (TEM) work comparing the atomic structure of a small number of actual radioactive sludges with the group’s synthetic version. (The TEM uses such a small sample that the radioactivity constitutes no danger.) In some cases the synthetic sludges contain nonradioactive elements from the periodic table that "weren’t exact matches for radioisotopes but provide useful insights into the way these things work," says Jim. Examples of this include substituting rhenium for technetium and neodymium for americium. Other elements, like strontium, cesium, and selenium, have nonradioactive isotopes that behave identically to the radioactive isotopes in the actual waste. Finally, the artificial sludges also contain nonradioactive components such as lead and cadmium present in the actual wastes that are of concern because of their heavy metal toxicity.

Sludge recipe

Step one of the process was to construct recipes for the different kinds of sludges that are likely to have resulted from the various chemical processes used to refine irradiated fuels over the past four decades. Then, says Jim, the next step was to make the sludges and see what sticks to each mix. The third step, to be completed this coming year, is to find in what ways these radionuclide surrogates are released.

"We are just completing the second year of a three-year grant from DOE’s Environmental Management Science Program, and it has become pretty clear that not all sludges would be expected to pick up the same radionuclides," says Jim. "For example, aluminum-rich sludges exhibit an affinity for components in the waste that travel as negatively charged ions in solution such as selenium and technetium. Iron-rich sludges fail to scavenge these elements. On the other hand, all the sludges scavenged cadmium, lead, barium, strontium, neodymium, and cobalt, and none absorbed a significant amount of cesium."

Current research efforts are focused on which of the chemical compounds in the sludges are responsible for retaining various radionuclides, and on assessing whether any of the radionuclide surrogates associated with the solid sludge would be released at a later date.

Other members of the research project team are, from Sandia, Patrick Brady, Pengchui Zhang, Sara Arthur, and Sheila Hutcherson (all 6118); and Kathy Nagy, University of Colorado.